Blende, which is an impure ZnS ore, is the main starting material for the production of Zn. The typical industrial practice encompasses an oxidative roasting step, producing ZnO together with sulphates or oxides of the impurities. In subsequent steps, the ZnO in roasted blende is brought into solution by leaching in neutral conditions or in weak acidic conditions, thereby producing Zn-depleted residues, respectively referred to in this description as neutral leach residue and as weak acid leach residue. However, during roasting, part of the Zn reacts with Fe, a typical impurity present in blende, and forms relatively insoluble zinc ferrite. The leach residues therefore contain, besides lead sulphate, calcium sulphate and other impurities, a sizeable fraction of Zn in the form of ferrite. According to present practice, the recovery of the Zn from ferrite requires a specific hydro-metallurgical residue treatment using high acid concentrations of 50 to 200 g/l H2SO4. A disadvantage of this acidic treatment is that besides Zn, almost all the Fe and also other impurities such as As, Cu, Cd, Ni, Co, Tl, Sb are dissolved. As even low concentrations of these elements interfere with the subsequent electrowinning of Zn, they must be removed from the zinc sulphate solution. While Cu, Cd, Co, Ni and Tl are precipitated by addition of Zn powder, Fe is typically discarded as hematite, jarosite or goethite through hydrolysis. Due to the danger of washout of heavy metals, these Fe-bearing residues have to be disposed off in a well-controlled landfill. Landfilling of such residues has however come under heavy environmental pressure, rendering the sustainability of the process questionable. Another drawback of the above treatment is the loss of metals such as In, Ge, Ag and Zn in the Fe-bearing residue.
An alternative treatment of the ferrite-bearing residues is applied in some plants, using Waelz kilns, which produce a slag, and a Zn and Pb containing fume. Similarly, a rotary flame-fired furnace of the Dorschel type can be used in a batch process. In still another approach, the leach residue is processed, using coke as fuel, in a half shaft blast furnace, producing a Zn and Pb containing fume, matte and slag. These pyro-metallurgical treatments generally result in an excellent recovery of Zn and Pb, and, for some of them, in a significant recovery of Ag, Ge and In.
These processes are however inadequate for modern zinc smelters, as they cannot be scaled up to large single-vessel operations. By this fact, they are not a cost efficient solution for today's Zn smelters.
In U.S. Pat. No. 2,932,566 oxidic zinciferous material is smelted with coke in a blast furnace and Zn is recovered from the furnace gases. In an example, fluxes are added to obtain a final slag with 61% FeO, 16% SiO2, 11.5% CaO and 3% Al2O3. In U.S. Pat. No. 4,072,503 Zn-, Fe- and Pb-bearing residues are fumed in a DC arc furnace, obtaining in one example a final slag with 43% FeO, 24% SiO2, 13% CaO, 6% MgO and 5% Al2O3. The smelting processes in above mentioned prior art documents take place in a packed bed or a still bath configuration, and not in an agitated bath or flash smelter at temperatures around 1300° C.
Recent literature mentions high temperature treatment of Zn-containing Fe-based secondary residues, such as EAF dusts. These temperatures are indeed needed to ensure a high Zn-fuming rate, down to low Zn content in the slag, in one single operation. In a known bath or flash smelting processes, the hitherto commonly used fayalite type of slag (2FeO.SiO2) is heated to well above its melting point (of about 1100° C.) during the metallurgical operation. Such strong superheating of the slag significantly shortens the lifetime of the refractory lining of the vessel. Using a water-cooled lining counters this effect, but at the prize of greatly increased heat losses. The batchwise operations in these smelters are therefore intentionally operated at low temperatures in order to preserve the bath lining and to limit the energy consumption; this however results in a discontinuous and slow fuming.
The primary aim of the invention is to provide a process for high-rate Zn-fuming, avoiding the corrosion of the vessel lining and limiting heat losses to a reasonable value.